Conventionally, an internal combustion engine (for example, an automobile engine) uses a spark plug for igniting an air-fuel mixture by means of spark discharge (may be referred to merely as “discharge”) as an ignition plug. In recent years, high output and low fuel consumption have been required of internal combustion engines. For example, a plasma jet ignition plug is known as an ignition plug which provides quick propagation of combustion and can reliably ignite a lean air-fuel mixture which is higher in air-fuel ratio than are air-fuel mixtures in conventional engines.
Such a plasma jet ignition plug has a small-volume discharge space called a cavity (chamber) formed as a result of a spark discharge gap between a center electrode and a ground electrode (external electrode) being surrounded by an insulator (housing) formed of ceramics or the like. When an air-fuel mixture is to be ignited by use of such a plasma jet ignition plug, first, a high voltage is applied between the center electrode and the ground electrode so as to perform spark discharge. By virtue of associated occurrence of dielectric breakdown, current can flow between the center electrode and the ground electrode at a relatively low voltage. Thus, through transition of a discharge state effected by further supply of energy between the center electrode and the ground electrode, plasma is generated within the cavity. The generated plasma is jetted through a communication hole (external-electrode hole) which is formed through the ground electrode, thereby igniting the air-fuel mixture (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-294257, hereinafter referred to as Patent Document 1).
Such plasma may assume one of various geometrical shapes; for example, the form of a flame column when it is jetted from the cavity (hereinafter the shape of such plasma will be referred to as a “flame shape”). Since flame-shaped plasma extends in the direction of jetting, the area of contact with the air-fuel mixture is large so that the plasma exhibits high igniting performance. A known technique for further improving the performance of igniting the air-fuel mixture is increasing the jetting length of the jetted plasma. In Patent Document 1 as well, an attempt is made to increase the jetting length of plasma by means of changing the volume and shape of the cavity in various manners.
However, in order to meet a demand for an improvement of fuel efficiency of an internal combustion engine, there has in turn been demanded an ignition plug which exhibits sufficient igniting performance even for a lean air-fuel mixture. There has been demanded an ignition plug in which plasma has an increased jetting length, and jets from the cavity more energetically to thereby ignite an air-fuel mixture more readily as described in Patent Document 1.